fix(mcp-server): make edit_diagram all-or-nothing and fix preview sync races

- edit_diagram applies nothing when any operation fails, rejects invalid or
  multi-cell new_xml, validates only the target page, and returns the
  current page XML on every rejection (including stale edits)
- Fix get_diagram reading the old diagram right after an AI write: the
  preview pushed its sync reply with a newer version than it was taken at
- Keep a user edit that loses the race with an AI write in history and
  tell the user in the preview
- Autofix removes only exact foreign tags (a stray <mxGraph/> deleted
  <mxGraphModel>), fixes tag case, drops orphan <mxPoint>s, and rejects
  unknown element names in model XML
- Edit empty and compressed pages; PNG exports use the page on screen;
  tag download exports; reload from the server after a page export
- Expand ~ in paths, tell the model when the browser sync timed out,
  use registerPrompt, require SDK ^1.31.0

packages/mcp-server/tests/server-wiring.test.ts

@@ -107,7 +107,7 @@ afterAll(() => {
 })
 
 describe("MCP server wiring", () => {
-    it("registers all nine multi-page tools", async () => {
+    it("registers all ten tools", async () => {
         const resp = await send("tools/list", {})
         expect(resp.error, JSON.stringify(resp.error)).toBeUndefined()
         const names: string[] = (resp.result?.tools ?? []).map(